indices en mysql

[alfa18]

mysql> describe t1;
+-------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
....................................
....................
..........

+-------+----------------------+------+-----+---------+----------------+
7 row in set (0.00 sec)

mysql> drop index `primary` on t1;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
mysql>

No entiendo porke me da ese error ¿Me lo podeis explicar? Por supuesto, solo tengo una columna auto_increment.

Gracias, saludos:D